Shaping is needed when the physical interface of the router runs at a higher speed than the path to the far end can support.
Two examples:
1. Frame relay circuits from a hub site to spoke remote sites. The hub port may be a full T1, and the spoke circuits perhaps 128kb. Without traffic shaping the hub site would burst data towards the spokes at the full T1 speed resulting in high packet loss in the frame network. With shaping the router would buffer the traffic and send it smoothly at a 128kb rate.
2. Service provider connectivity with an ethernet interface handoff that has less than port speed throughput. If you are paying for 3mb TLS service the provider will police and throw away packets that exceed 3mb. If you shape at 3mb the packets will be buffered and delayed resulting in better performance.
The answer to your specific question depends on what is connected to the interfaces. If the serial is a full T1 private line circuit to another router, and the gig port connects to a LAN switch then you don't need any traffic shaping.
http://www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_configuration_guide_chapter09186a00800c60d1.html
About Traffic Shaping
Traffic shaping allows you to control the traffic going out an interface in order to match its flow to the speed of the remote, target interface and to ensure that the traffic conforms to policies contracted for it. Thus, traffic adhering to a particular profile can be shaped to meet downstream requirements, thereby eliminating bottlenecks in topologies with data-rate mismatches.
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